Systems and methods for implementing an interaction between a laser shaped as line beam and a film deposited on a substrate

ABSTRACT

A laser crystallization apparatus and method are disclosed for selectively melting a film such as amorphous silicon that is deposited on a substrate. The apparatus may comprise an optical system for producing stretched laser pulses for use in melting the film. In still another aspect of an embodiment of the present invention, a system and method are provided for stretching a laser pulse. In another aspect, a system is provided for maintaining a divergence of a pulsed laser beam (stretched or non-stretched) at a location along a beam path within a predetermined range. In another aspect, a system may be provided for maintaining the energy density at a film within a predetermined range during an interaction of the film with a shaped line beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 12/386,771, filed on Apr. 21, 2009, entitled “SYSTEMS ANDMETHODS FOR IMPLEMENTING AN INTERACTION BETWEEN A LASER SHAPED AS A LINEBEAM AND A FILM DEPOSITED ON A SUBSTRATE,” which is a continuation ofU.S. application Ser. No. 11/138,001, filed on May 26, 2005, entitled“SYSTEMS AND METHODS FOR IMPLEMENTING AN INTERACTION BETWEEN A LASERSHAPED AS A LINE BEAM AND A FILM DEPOSITED ON A SUBSTRATE, which is acontinuation-in-part of U.S. application Ser. No. 10/712,545, filed onNov. 13, 2003 and titled, “LONG DELAY AND HIGH TIS PULSE STRETCHER”which is a continuation-in-part of U.S. application Ser. No. 10/141,216,filed on May 7, 2002, now U.S. Pat. No. 6,693,939, and titled, “LASERLITHOGRAPHY LIGHT SOURCE WITH BEAM DELIVERY,” the disclosures of each ofwhich are hereby incorporated by reference herein.

Also, U.S. application Ser. No. 11/138,001, of which U.S. applicationSer. No. 12/386,771 is a continuation application, is also acontinuation-in-part of U.S. application Ser. No. 10/781,251, titled“VERY HIGH ENERGY, HIGH STABILITY GAS DISCHARGE LASER SURFACE TREATMENTSYSTEM,” filed on Feb. 18, 2004.

Also, U.S. application Ser. No. 11/138,001, of which U.S. applicationSer. No. 12/386,771 is a continuation application is also acontinuation-in-part of U.S. application Ser. No. 10/425,361, filed onApr. 29, 2003 and titled, “LITHOGRAPHY LASER WITH BEAM DELIVERY AND BEAMPOINTING CONTROL.”

FIELD OF THE INVENTION

The present invention relates to systems and methods for positioning afilm for interaction with a laser shaped as a line beam and forcontrolling parameters of the shaped line beam, for example, to melt anamorphous silicon film, for example, to crystallize the film for thepurpose of manufacturing thin film transistors (TFT's).

BACKGROUND OF THE INVENTION

Laser crystallization of an amorphous silicon film that has beendeposited on a substrate, e.g., glass, represents a promising technologyfor the production of material films having relatively high electronmobilities. Once crystallized, this material can then be used tomanufacture thin film transistors (TFT's) and in one particularapplication, TFT's suitable for use in relatively large liquid crystaldisplays (LCD's). Other applications for crystallized silicon films mayinclude Organic LED (OLED) and System on a Panel (SOP). In morequantitative terms, high volume production systems may be commerciallyavailable in the near future capable of quickly crystallizing a filmhaving a thickness of about 90 nm and a width of about 700 mm or longer.This process may be performed using a pulsed laser that is opticallyshaped to a line beam, e.g., a laser that is focused in a first axis,e.g., the short axis, and expanded in a second axis, e.g., the longaxis. Typically, the first and second axes are mutually orthogonal andboth axes are substantially orthogonal to a central ray traveling towardthe film. An exemplary line beam for laser crystallization may have abeam width of less than about 20 microns and a beam length of about 700mm. With this arrangement, the film can be scanned or stepped in adirection parallel to the beam width to sequentially melt andcrystallize a film having a substantial length, e.g., 700 mm or more.

In some cases, it may be desirable to ensure that each portion of thesilicon film is exposed to a laser energy density that is controlledwithin a preselected energy density range during melting. In particular,energy density control within a preselected range is typically desiredfor locations along the shaped line beam, and a somewhat constant energydensity is desirable as the line beam is scanned relative to the siliconfilm. High energy density levels may cause the film to flow resulting inundesirable “thin spots”, a non-flat surface profile and poor grainquality. This uneven distribution of film material is often termed“agglomeration” and can render the crystallized film unsuitable forcertain applications. On the other hand, low energy density levels maylead to incomplete melting and result in poor grain quality. Bycontrolling energy density, a film having substantially homogeneousproperties may be achieved.

One factor that can affect the energy density within an exposed film isthe spatial relationship of the thin film relative to the pulsed laser'sdepth of focus (DOF). This DOF depends on the focusing lens, but for atypical lens system configured to produce a line beam having a 20 micronbeam width, a good approximation of DOF may be about 20 microns.

With the above in mind, it is to be appreciated that a portion of thesilicon film that is completely within the laser's DOF will experience adifferent energy density than a portion of the silicon film that is onlypartially within the laser's DOF. Thus, surface variations of thesilicon film, the glass substrate and the vacuum chuck surface whichholds the glass substrate, even variations as small as a few microns, ifunaccounted for, can lead to unwanted variations in energy density fromone film location to another. Moreover, even under controlledmanufacturing conditions, total surface variations (i.e., vacuumchuck+glass substrate+film) can be about 35 microns. It is to beappreciated that these surface variations can be especially problematicfor focused thin beam having a DOF of only about 20 microns.

In addition to surface variations, unwanted movements of the filmrelative to the shaped line beam can also lead to variations in energydensity. For example, small movements can occur during stage vibrations.Also, an improper alignment of the stage relative to the shaped linebeam and/or an improper alignment of the stage relative to the scanplane can result in an unwanted energy density variation.

Other factors that can lead to a variation in energy density from onefilm location to another can include changes in laser outputcharacteristics during a scan (e.g., changes in pulse energy, beampointing, beam divergence, wavelength, bandwidth, pulse duration, etc).Additionally, the location and stability of the shaped line beam and thequality of the beam focus (e.g., shape) during a scan can affect energydensity uniformity.

With the above in mind, Applicants disclose several systems and methodsfor implementing an interaction between a shaped line beam and a filmdeposited on a substrate.

SUMMARY OF THE INVENTION

Systems and methods are disclosed for producing pulses having pulsecharacteristics suitable for an interaction with a film depositedsubstrate as a shaped beam defines a short axis and a long axis. In oneaspect of an embodiment of the present invention, a system and methodare provided for stretching a laser pulse. The system may comprise abeam splitter for directing a first portion of the pulse along a firstbeam path and a second portion of the pulse along a second delaying beampath; and a plurality of reflective elements positioned along thedelaying beam path and arranged to invert the second beam portion andcooperate with the beam splitter to place at least a portion of theinverted second beam portion for travel on the first beam path.

In another aspect of an embodiment of the present invention, a systemand method are provided for maintaining a divergence of a pulsed laserbeam at a location along a beam path within a predetermined range. Thissystem may comprise an adjustable beam expander; an instrument formeasuring divergence and generating a signal indicative thereof; and acontroller responsive to the signal to adjust the beam expander andmaintain the divergence of the pulsed laser beam within thepredetermined range.

In yet another aspect of an embodiment of the present invention, a lasercrystallization apparatus and method are provided for selectivelymelting a film disposed on a substrate. The apparatus may comprise alaser source producing a pulsed laser output beam; an optical systemstretching pulses in the laser output beam to produce a pulse stretcheroutput; and an optical arrangement producing a line beam from the pulsestretcher output.

In still another aspect of an embodiment of the present invention, asystem and method are provided for maintaining the energy density at afilm within a predetermined range during an interaction of the film witha shaped beam. This system may comprise an autofocus sensor formeasuring a distance between the film and a focusing lens; and acontroller using the measurement to adjust a light source parameter tomaintain the energy density at the film with the predetermined range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of the primary components of an exemplaryproduction system for crystallizing an amorphous silicon film;

FIG. 2 shows an apparatus for determining whether a line beam is focusedat a film deposited on a substrate;

FIG. 3A shows a graphical representation showing intensity variation asa function of short axis beam width for three exemplary beams: a firstbeam having a best focus in the sampled plane, a second beam having abest focus ten microns from the sample plane and a third beam having abest focus that is fifteen microns from the sampled plane;

FIG. 3B shows a graphical representation showing energy density as afunction of lateral growth length and indicates regions where partialmelting and agglomeration may occur;

FIG. 4 shows a perspective view of a vacuum chuck assembly for holding aworkpiece during an interaction with a line beam;

FIGS. 5A-5Q are schematic plan views showing a system, and illustratingits use, for positioning a silicon film for interaction with a linebeam;

FIG. 6 shows a schematic view of a portion of the system shown in FIG. 1illustrating an aspect of an embodiment of the present invention;

FIG. 7 shows a detailed view of a six-mirror pulse stretcher;

FIG. 8 shows a plot of intensity vs. time for a stretched and anunstretched pulse;

FIG. 9 shows a plot of intensity vs. vertical width illustrating theincrease in vertical uniformity of a stretched pulse as compared to anunstretched pulse; and

FIG. 10 shows an actively controllable beam expander that isindependently adjustable in two axes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially to FIG. 1, there is shown a schematic, not to scale,view of the primary components of an exemplary production system,designated generally system 10, for crystallizing an amorphous siliconfilm 12. As shown, the system 10 may include a laser source 20 forgenerating a pulsed laser beam, a pulse stretcher 22 for increasingpulse duration and a beam delivery unit 24 which may have a mechanism toactively steer the beam and/or an active beam expander. The system 10may further include a stabilization metrology module 26 for measuringone or more beam characteristics, e.g., wavefront and/or beam pointing,and generating control signals for use by the active steering unitand/or the active beam expander. System 10 may also include an opticsmodule 28 for beam homogenization, beam shaping and/or beam focusing,and a moveable stage system 30 for holding and positioning a siliconfilm 12 that has been deposited on a substrate 32, which can be forexample, glass.

In overview, the system 10 shown in FIG. 1 and described in greaterdetail below can be configured to generate a focused thin beam 34, e.g.line beam, having a width at the film 12 of about 20 microns or less(short axis) and a length of 700 mm or more (long axis) and a depth offocus (DOF) of about 10-20 microns. Each pulse of the focused thin beamcan be used to melt a strip of amorphous silicon.

Thereafter, the strip crystallizes. In particular, the stripcrystallizes in a lateral growth process wherein grains grow in adirection parallel to the short axis. Grains grow inward from the edges(parallel to the short axis) and meet creating a ridge (a so-calledgrain boundary protrusion) along the center of the strip which extendsout of the plane of the silicon film. The stage is then moved, eitherincrementally or continuously, to expose a second strip that is parallelto and overlaps a portion of the first strip. During exposure, thesecond strip melts and subsequently re-crystallizes. An overlapsufficient to re-melt the ridge may be used. By re-melting the ridge, aflat film surface (e.g., peak-to-peak value of ˜15 nm) may bemaintained. This process, which is hereinafter referred to as thin-beam,sequential lateral solidification (tb-SLS) is typically repeated untilthe entire film is crystallized.

FIG. 2 shows an apparatus for determining whether a thin beam pulsedlaser 34 is properly focused at a silicon film 12 deposited on asubstrate 32. Portions of the optics module 28 (see FIG. 1) may includea short axis field stop 36 and a short axis focusing optic 37.Typically, the beam is initially focused at the field stop 36 and thenimaged to produce an intensity profile as shown in FIG. 3A (plot 62) atthe film 12. FIG. 3A shows a profile (plot 62) for a field stop 36constructed as a slit having a small dimension in the short axis. Thisarrangement may be used to produce the profile shown in FIG. 3A whichhas a beam width (FWHM) of about 13 gm, an intensity uniformity betterthan 5% along the flat top of the profile, and steep edge slopes thatmay be less than about 3 um between the 10% and 90% of full intensity.Beams having a width between about 5 and 10 um may also be used. Asingle-edge (i.e., knife-blade) may be used in place of the slit at thefield stop to produce a beam profile having a steep trailing edge slope(i.e., the edge corresponding to the material that will not be re-meltedduring the tb-SLS process) while leaving the leading edge unaffected.Although shown as a single lens, it is to be appreciated that thefocusing optic 37 may include a number of optical components including,but not limited to various types of lenses.

FIG. 2 shows that the beam 36 is non-collimated and diverges from thefield stop 36 to the film 12 in the long axis 38. As indicated above,the length of the beam 36 in the long axis 38 may be about 700 mm orlonger. On the other hand, as illustrated in FIG. 1, the beam 36 isfocused in the short axis 40 by the optics module 28 which may includethe focusing optic 38. With this structural arrangement, light 42 thathas reflected from the film 12 continues to diverge from the opticalaxis 44 and can be analyzed by a detecting system to determine whetherthe beam 36 is adequately focused in the short axis 40 (shown in FIG.1).

Continuing with reference to FIG. 2, the detecting system may include afull reflection mirror 46 which directs reflected light 42 to an imageplane 48. A magnifying lens 50 is positioned to provide a magnifiedimage of the image plane 48 at the camera 52. For the detecting system,the image plane 48 is positioned such that the distance traveled byreflected light 42 from the film 12 to the image plane 48 isapproximately equal (e.g. equal within acceptable tolerances for thepertinent art) to the distance the same light had traveled from theshort axis field stop 36 to the film. A camera output can then be usedto adjust one or more system variables to improve the focus at film 12or, as discussed in more detail below, alter the energy density at thefilm 12. For example, the stage 30 can be moved relative to the focusingoptic 37 to adjust the focus.

In some cases, as shown in FIG. 2, it may be desirable to include asecond detection system, similar to the one described above, having amirror 54, magnifying lens 56 and camera 58. In combination, the twodetecting systems can be used to simultaneously determine whether lightis focused (in the short axis) at both ends of the beam in the long axis44. One noteworthy functional aspect for the detecting systems shown inFIG. 2, is that the detecting systems, and in particular the mirrors 46,54, are positioned such that they do not interfere with light travelingfrom the short axis field stop 36 to the substrate 12. Moreover, thisarrangement allows the focus of the thin beam to be analyzed andadjusted during an exposure of the film 12.

FIG. 3A shows a graphical representation showing intensity variation asa function of short axis beam width for a focused beam (plot 62), a beamthat is ten microns out-of-focus (plot 64) and a beam that is fifteenmicrons out-of-focus (plot 66). Note; the plots shown are for a focusinglens having a numerical aperture (NA) of approximately 0.15. Aninteresting feature of these plots is that all of the plots 62, 64, 66have relatively steep sidewalls. These steep sidewalls are a result ofthe optical arrangement shown in FIG. 2 in which a short-axis field stop36 is used. As such, FIG. 3A shows that the variation in beam intensityas a beam goes out of focus is more pronounced than the correspondingchange in beam width. As indicated earlier in the Background Section ofthis document, it may be desirable to maintain energy density within apre-selected range at the film 12. More specifically, energy densitycontrol at the film 12 may be achieved over a range of focus conditionsby varying characteristics of the laser pulse, e.g., pulse energy, withonly a small change in beam width.

With the above in mind, Applicants disclose a system and method formaintaining energy density within a preselected range at the film 12,e.g., by altering a pulse characteristic to compensate for a change infocal condition. This change in focal condition can occur, for example,during a scan movement of the stage 30 relative to the laser beam. Ingreater detail, an energy density obtained with a slightly out-of focusbeam (e.g., plot 66) may be chosen as the target energy density. Withthis target, a focal condition is measured, for example, using thedetecting system shown in FIG. 2. It is to be appreciated that alternatemethods to determine the focal condition can be used to includeautofocus sensors (active or passive) or other suitable techniquescapable of measuring a distance between the film 12 and the focusingoptic 37 (see FIG. 2). Once the focal condition has been measured, apulse characteristic such as pulse energy may be altered to reach thetargeted energy density. Thus, if the measurement indicates that anout-of-focus condition exists, a first pulse energy, E1, correspondingto the target energy density for the out-of-focus condition is used. Onthe other hand, when the measurement indicates that the film 12 iswithin the DOF, a second pulse energy, E₂, corresponding to the targetenergy density for the focus condition is used, with E₁<E₂.

FIG. 3B shows a graphical representation showing energy density as afunction of lateral growth length for a 50 nm thick Si film andindicates regions where partial melting and agglomeration may occur.FIG. 3 also shows that the energy range for the lateral growth may bequite wide (approx. 450 mJ/cm² and 820 mJ/cm²) with the lateral growthlength increasing proportionally with energy density. Larger lateralgrowth length, in turn, may be beneficial by allowing a larger scanpitch (and higher throughput) while still re-melting the center ridge.

Several methods can be used to adjust the pulse energy, as desired, andin some cases on a pulse-to-pulse basis. For example, for an Excimerlaser source, the discharge voltage can be varied to achieve apre-selected pulse energy. Alternatively, an adjustable attenuator canbe positioned along the laser beam's beam path to selectively alterpulse energy. For this purpose, any device known in the art for reducingpulse energy including, but not limited to, filters and pulse trimmersmay be used. Other pulse characteristics that can be altered tocompensate for focus condition to maintain energy density within apreselected range at different locations at the film 12 may include, butare not necessarily limited to, pulse spectrum (i.e., wavelength) usingfor example an adjustable line narrowing module or a line selectionmodule. Alternatively, an adaptive optic capable of fast focus controlcan be used as the focusing optic 37 responsive to a measured focalcondition in a controlled feedback loop.

FIGS. 4 and 5A-Q illustrate a system and corresponding method of use toposition a film 12 for interaction with a line beam focused from a lasersource. As shown in FIG. 4, an exemplary positioning arrangement caninclude a vacuum chuck 100 having a substantially planar surface 101(also shown in FIG. 5A), e.g. planar within manufacturing tolerances forthe pertinent art, that is positioned on, or is formed as an integralpart of, a so-called ZPR table 102 (see FIG. 5A), which may comprise amoveable wedge assembly. As best seen with cross-reference to FIGS. 4and 5A, the ZPR table 102 can be functionally capable of independentlymoving the vacuum chuck 100, back and forth in a Z direction, a Pitchdirection wherein the chuck 100 is rotated about an X-axis, and a Rolldirection in which the chuck 100 is rotated about a Y-axis. FIG. 5A alsoshows, albeit schematically, that the system may include an X-stage 104for moving the vacuum chuck 100 back and forth in an X direction and aY-stage 106 for moving the vacuum chuck 100 back and forth in a Ydirection. In a typical exemplary setup, X, Y and Z are three mutuallyorthogonal axes. As shown, both stages 104, 106 can be moveable relativeto a stable reference block 108, e.g. granite block, which defines asubstantially planar reference surface 110 (e.g. planar withinmanufacturing tolerances for the pertinent art). Typically, air bearingsmay be employed between the stages 104, 106 and granite block 108.

As best seen in FIG. 5B, the system may include a plurality of opticalsensors, which for the embodiment shown are three autofocus sensors 112a-c, e.g., active or passive autofocus sensors that are fixedly mountedrelative to the granite block 108 via overhead housing 114. As shown,the three autofocus sensors 112A-C are spaced apart along the X-axis.Typically, they can be positioned along a line parallel to or on theX-axis. Moreover, as shown, each autofocus sensor 112 a-c is oriented tomeasure a distance, such as distance 116, parallel to the Y-axis,between the respective autofocus sensors 112 a-c and the surface 101.This, in turn, provides a distance, parallel to the Y-axis, between thesurface 101 and the reference plane 110. Although three optical sensorsare shown, it is to be appreciated that a system having more than threeand as few as one optical sensor may be employed to perform some or allof the functional aspects detailed below.

FIGS. 5B-5E illustrate how the system can determine a roll angle, a,between the surface 101 and the reference plane 110. Specifically,beginning with FIG. 5B, it can be seen that a first measurement(distance 116) can be made between the autofocus sensor 112A and thesurface 101 with the table 102 at a first position along the X-axis.Next, as shown in FIG. 5C, the table 102 can be translated along theX-axis by activation of the X-stage to position the table at a secondposition along the X-axis. At this second position, a second distancemeasurement can be made between the autofocus sensor 112A and thesurface 101. Although two measurements are sufficient, FIG. 5D showsthat the system may perform a third measurement, with the table at athird position along the X-axis. These measurements can then beprocessed in an algorithm to determine a roll angle, a, between thesurface 101 and the reference plane 110, as shown in FIG. 5E. Note asimilar procedure (not illustrated) can be performed by moving the table102 along the Y-axis to determine an incline angle of the surface 101relative to the reference plane.

Once the roll angle, a, (and, if desired, an incline angle) have beendetermined, the ZPR table 102 can be selectively activated to move thesurface 101until it is substantially parallel to the reference plane110, as shown in FIG. 5F. At this point, a stage coordinate system maybe established. In addition, as shown in FIG. 5G, the three autofocussensors 112 a-c may be calibrated for distance to the surface 101 andlinearity over the measuring range. With this calibration, the surface101 can be used as a reference (e.g., an autofocus reference plane) forfuture measurement.

In one implementation of the system, the spatial position andorientation of a focused line beam of a laser can be determined. Anexemplary focused beam which can be characterized by a substantiallylinear beam axis 118, is depicted as a dashed line in FIG. 5H. For thesystem shown, pulsed laser light arrives at the beam axis 118 from aboveand in front of the overhang housing 114. Moreover, a portion of the ZPRtable 102 extends outwardly along the Y-axis from the overhang housing114, such that at least a portion of the surface 101 of the table 102may be exposed to the pulsed laser.

As further shown in FIG. 5H, the system may comprise a detector, whichfor the embodiment shown can be a line beam camera 120, for measuringpositions for a plurality of the thin beam focal locations (e.g.,locations of best focus). More particularly, as shown, the line beamcamera 120 can be mounted on the ZPR table 102, and accordingly, may bemoveable therewith. It is to be appreciated that an arrangement having aplurality of line beam cameras (not shown) may be used to measure aplurality of line beam focal locations without movement of the X-stage.

FIGS. 5I-5L illustrate how the system with one camera 120 can determinethe spatial position of the beam axis 118 and a relative angle, φ,between the surface 101 and the beam axis 118. Specifically, beginningwith FIG. 51, it can be seen that a first measurement can be made by thecamera 120 representing (distance 122 a), parallel to the Y-axis,between the beam axis 118 and the reference plane 110, with the table102 at a first position along the X-axis. Next, as shown in FIG. 5J, thetable 102 can be translated along the X-axis by activation of theX-stage 104 to position the table at a second position along the X-axis.At this second position, a second distance measurement 122 a, parallelto the Y-axis, may be made by the camera 120. Although two measurementsmay be sufficient in some cases, FIG. 5K shows that the system may,e.g., perform a third measurement (distance 122 c), with the table 102at a third position along the X-axis. These measurements can then beprocessed in an algorithm to determine a relative angle, φ, between thesurface 101 and the beam axis 118, as shown in FIG. 5L.

Once the relative angle, φ, between the surface 101 and the beam axis118 has been determined, the ZPR table 102 can be selectively activatedto move and orient the table 102 into an alignment wherein the surface101 is substantially parallel (e.g. parallel within acceptabletolerances for the pertinent art) to the beam axis 118, as shown in FIG.5M. Once aligned, FIG. 5N shows that the autofocus sensors 112 a-c maybe used to measure the position of surface 101 (i.e., the autofocusreference plane) and calibrate the autofocus sensors 112 a-c on theautofocus reference plane. This then establishes a Laser/Stagecoordinate system.

FIG. 50 shows that the glass substrate 32 and deposited film 12 may nowbe positioned on the vacuum chuck (i.e., surface 101). As shown there,the X-stage 104 can be activated to move the table 102 to a favorableposition to facilitate positioning of the film 12 on the surface 101.With the film positioned on the table 102, the table 102 can be movedfor interaction with the autofocus sensors 112 a-c as shown in FIG. 5P.There, the autofocus sensors 112 a-c may be used to determine the heightof the film 12. With the height of the film 12 known, the ZPR table 102can be activated to move the film 12 to within the depth of focus (DOF)of the focused, line beam as shown in FIG. 5Q. With the film 12 withinthe DOF of the laser, the laser can be activated to expose and melt astrip of the film 12, for example, as part of a thin beam, sequentiallateral solidification (tb-SLS) process described above.

In another aspect of an embodiment of the present invention, the systemshown in FIGS. 5A-5Q may be used to compensate for a film 12 having animperfect, non-planar surface. This variation in surface profile mayresult from dimensional imperfections in the film 12, the glasssubstrate 32 and/or the vacuum chuck surface 101. By compensating forthe variations in surface profile, a substantially constant energydensity can be maintained at different locations of the film 12. Forthis purpose, as shown in FIG. 5P, the method can include the first stepof using the three autofocus sensors 112 a-c to determine threerespective distances, parallel to the Y axis, between the sensors andthe film 12. Using the line beam camera 120, the ZPR table 102 may bemanually adjusted (by varying Z, pitch and roll) to position the surface101 along a line of best focus (i.e., beam axis 118). Next, therespective distances between each sensor 112 a-c and the film 12 can bestored as reference distances, resulting in three coordinate points onthe film 12. A linear fit through these three coordinate points can beused to determine a calculated line of best focus (axis 118). Duringexposure, as the film 12 is scanned along the Y-axis, distances to thefilm 12 can be measured by e.g., the three autofocus sensors 112 a-c,resulting in three new coordinate points. A best-fit line can then becalculated that passes through these new coordinate points and the ZPRtable 102 can be adjusted via computer control to align the table 102such that the best-fit line is substantially co-incident (e.g.,coincident within acceptable tolerances for the pertinent art) to thecalculated line of best focus (axis 118).

FIG. 6 shows in further detail a portion of the system 10 shown inFIG. 1. Specifically, FIG. 6 shows an exemplary embodiment having a twochamber, excimer laser 20. It is to be appreciated that other types oflaser sources could be used in the system 10, to include solid statelasers, excimer lasers having one chamber, excimer lasers having morethan two chambers, e.g., an oscillator chamber and two amplifyingchambers (with the amplifying chambers in parallel or in series), or asolid state laser that seeds one or more excimer amplifying chambers.Other designs are possible. Further details for a two chamber lasersource 20 shown in FIG. 6, can be found in U.S. application Ser. No.10/631,349, entitled CONTROL SYSTEM FOR A TWO CHAMBER GAS DISCHARGELASER, filed on Jul. 30, 2003, U.S. Ser. No. 10/356,168, entitledAUTOMATIC GAS CONTROL SYSTEM FOR A GAS DISCHARGE LASER, filed on Jan.31, 2003, U.S. Ser. No. 10/740,659, entitled METHOD AND APPARATUS FORCONTROLLING THE OUTPUT OF A GAS DISCHARGE MOPA LASER SYSTEM, filed onDec. 18, 2003, U.S. Ser. No. 10/676,907, entitled GAS DISCHARGE MOPALASER SPECTRAL ANALYSIS MODULE filed on Sep. 30, 2003, U.S. Ser. No.10/676,224, entitled OPTICAL MOUNTINGS FOR GAS DISCHARGE MOPA LASERSPECTRAL ANALYSIS MODULE, filed Sep. 30, 2003, U.S. Ser. No. 10/676,175,entitled GAS DISCHARGE MOPA LASER SPECTRAL ANALYSIS MODULE, filed Sep.30, 2003, U.S. Ser. No. 10/631,349, entitled CONTROL SYSTEM FOR A TWOCHAMBER GAS DISCHARGE LASER, filed Jul. 30, 2003, U.S. Ser. No.10/627,215, entitled VERY NARROW BAND, TWO CHAMBER, HIGH REP-RATE GASDISCHARGE LASER, filed on Jul. 24, 2003, U.S. Ser. No. 10/607,407,entitled METHOD AND APPARATUS FOR COOLING MAGNETIC CIRCUIT ELEMENTS,filed on Jun. 25, 2003, U.S. Ser. No. 10/922,692, entitled TIMINGCONTROL FOR TWO-CHAMBER GAS DISCHARGE LASER SYSTEM, filed on Aug. 20,2004, U.S. Pat. No. 6,625,191, entitled HIGH REP RATE MOPA LASER SYSTEM,and U.S. Pat. No. 6,567,450, entitled BASIC MODULAR MOPA LASER SYSTEM,the disclosures of all of which are hereby incorporated by referenceherein.

In overview, FIG. 6 shows that the two chamber laser source 20 mayinclude a master oscillator 208 and a power amplifier 210, andaccordingly, is often referred to as a so-called MOPA laser source. Inone implementation of the tb-SLS process described above, a 6 Khz (6000pulses per second) MOPA laser may be used with pulse energies ofapproximately 150 mJ. With this arrangement, a 730 mm×920 mm film may beprocessed (with 60 percent overlap) in about 75 seconds.

The master oscillator 208 and the power amplifier 210 each comprise adischarge chamber which may contain two elongated electrodes, a lasergas, e.g., XeCI, XeF, ArF or KF, a tangential fan for circulating thegas between the electrodes and one or more water-cooled finned heatexchangers (not shown). The master oscillator 208 produces a first laserbeam 214A which can be amplified by, e.g., two passes through the poweramplifier 210 to produce laser beam 214B. The master oscillator 208 cancomprise a resonant cavity formed by output coupler 208A and linenarrowing module 208B both of which are described in detail in theapplications and patents referenced earlier. The gain medium for masteroscillator 208 may be produced between two electrodes, each about thirtyto fifty centimeters in length and contained within the masteroscillator discharge chamber.

Power amplifier 210 may comprise a discharge chamber similar to thedischarge chamber of the master oscillator 208 providing a gain mediumbetween two elongated electrodes. However, unlike the master oscillator208, the power amplifier 210 typically has no resonant cavity and thegas pressure can, in general, be maintained higher than that of themaster oscillator 208. The MOPA configuration shown in FIG. 6 permitsthe master oscillator 208 to be designed and operated to maximize beamquality parameters such as wavelength stability, and to provide a verynarrow bandwidth; whereas the power amplifier 210 may be designed andoperated to maximize power output.

The output beam 214A of the master oscillator 8 can be amplified by,e.g., two passes through power amplifier 210 to produce output beam214B. The optical components to accomplish this can be contained inthree modules which Applicants have named: master oscillator wave frontengineering box, MO WEB, 224, power amplifier wavefront engineering box,PA WEB, 226 and beam reverser, BR, 228. These three modules along withline narrowing module 208B and output coupler 208A may all be mounted ona single vertical optical table independent of discharge chamber 208Cand the discharge chamber of power amplifier 210. With this arrangement,chamber vibrations caused by acoustic shock and fan rotation may besubstantially isolated from the optical components.

The optical components in the line narrowing module 208B and outputcoupler 208A are described in greater detail in the applications andpatents referenced above. In overview, the line narrowing module (LNM)208B may comprise a three or four prism beam expander, a very fastresponse tuning mirror and a grating disposed in Litrow configuration.The output coupler 208A may be a partially reflecting mirror whichtypically reflects about 20 percent of the output beam for KrF systemsand about 30 percent for ArF systems. The remaining non-reflected lightpasses through output coupler 208 and into a line center analysis module(LAM) 207. From the LAM 207, light may pass into the MO WEB 24. The MOWEB may comprise a total internal reflection (TIR) prism (or firstsurface mirror with a high reflection coating) and alignment componentsfor precisely directing the output beam 214A into the PA WEB 226.

The PA WEB 226 may comprise a TIR prism (or first surface mirror with ahigh reflection coating) and alignment components for directing a laserbeam 214A into a first pass through power amplifier gain medium. Thebeam reverser module 228 may comprise a two-reflection beam reversingprism which relies on total internal reflection and therefore requiresno optical coatings. Alternatively, the beam reverser 228 may be a fullreflection mirror. In either case, the beam reverser 228 may beadjustable in response to a control signal from a metrology device,e.g., SMM 26, to direct the partially amplified beam on a pre-selectedbeam path back through the power amplifier gain medium. In particular,the beam reverser may be adjusted to correct beam pointing error and, asdiscussed below, to reduce the beam divergence of the beam exiting thepulse stretcher 22.

After reversal in the beam reversing module 228, the partially amplifiedbeam 214A can make another pass through the gain medium in poweramplifier 210 and exit through spectral analysis module 209 and PA WEB226 as power amplifier output beam 214B. From the PA WEB 226, the beamenters, e.g., a six-mirror pulse stretcher 22 which, as detailed below,may increase pulse duration, reduce beam intensity variations across thebeam section (i.e., average or smooth out the intensity profile), andreduce beam coherence. By increasing pulse duration, the peak intensityof each laser pulse is reduced while maintaining pulse energy. For thesystem 10 shown in FIG. 1, the optical components in the optics module28 may comprise relatively large lenses, which are difficult andexpensive to fabricate. These expensive optical components are oftensubject to degradation resulting from billions of high intensityultraviolet pulses. Moreover, optical damage is known to increase withincreasing intensity (i.e., light power (energy/time) per cm.sup.2 ormJ/ns-cm.sup.2) of the laser pulses. Thus, reducing pulse intensity byincreasing pulse duration may increase the life of these opticalcomponents. Moreover, an increase in pulse duration may be beneficial inthe crystallization process. Instead of, or in addition to the sixmirror pulse stretcher 22, one or more of the pulse stretchers disclosedin co-pending U.S. application Ser. No. 10/712,545, filed on Nov. 13,2003 and titled, “LONG DELAY AND HIGH TIS PULSE STRETCHER” may be usedto create stretched pulses for use in the thin beam sequential lateralsolidification (tb-SLS) process described herein, and in particular,pulse stretchers having an output pulse of 200 ns time integrated square(TIS) may be used. U.S. application Ser. No. 10/712,545 is herebyincorporated by reference herein.

FIG. 7 shows a more detailed view of the six-mirror pulse stretcher 22showing the beam paths through pulse stretcher 22. A beam splitter 216can be selected to reflect a predetermined percent of the poweramplifier output beam 214B into a delay path created by six focusingmirrors 320A, 320B, 320C, 320D, 320E and 320F. The remaining light istransmitted through the beam splitter 216. It is to be appreciated thatthe beamsplitter's reflect/pass characteristic may affect the outputpulse duration and/or output pulse shape. For the stretcher 22, eachmirror 320A-F may be a focusing mirror, e.g., a concave sphericalmirror. Typically, to facilitate alignment of the pulse stretcher 22,one or more of the six mirrors may be adjustable, e.g., a tip/tiltadjustment.

As shown in FIG. 7, reflected light from the beam splitter 216 maytravel unfocused (i.e., substantially collimated) along path 301A tomirror 320A which then focuses a reflected portion along path 301B topoint 302 which is located midway between mirror 320A and mirror 320B.The beam then expands and may be reflected from mirror 320B, whichconverts the expanding beam into a parallel (i.e., substantiallycollimated) beam and directs it along path 301C to mirror 320C. Mirror320C, can then focus a reflected portion along path 301D to point 304which may be located midway between mirror 320C and mirror 320D. Thebeam then expands and can be reflected from mirror 320D which convertsthe expanding beam into a parallel (i.e., substantially collimated) beamand directs it along path 301E to mirror 320E. Mirror 320E, can thenfocus a reflected portion along path 301F to point 306 which is locatedmidway between mirror 320E and mirror 320F. The beam may then expand andbe reflected from mirror 320F which converts the expanding beam into aparallel (i.e., substantially collimated) beam and directs it along path301G to beam splitter 216. At the beam splitter 216, the beam frommirror 320F may be reflected onto path 301H where it joins the portionof the pulse that is transmitted through the beam splitter 216.Together, the transmitted and delayed pulse portions establish a pulsestretcher beam output 214C, as shown. The stretched pulse 400 is plottedas intensity vs. time in FIG. 8 and can be compared with the shape ofthe power amplifier output pulse 402 (unstretched pulse) which is alsoplotted in FIG. 8. For the stretched pulse shown, the pulse may beshaped having two large, approximately equal peaks with smallerdiminishing peaks following in time after the first two peaks. It is tobe appreciated that the shape of the stretched pulse can be modified byusing a beam splitter having a different reflectivity.

FIG. 7 shows that the delayed beam may be focused and expanded threedifferent times. Because of this odd (i.e., non-even) number of focusingsteps, the delayed beam is inverted (both horizontally and vertically)with respect to the portion of the pulse that is transmitted through thebeam splitter 216. Hence, the output beam 214C from the six-mirror pulsestretcher 22 may include a combination, or mixed beam. This mixing mayreduce intensity variations. The pulse stretcher 22 may also reduce beamcoherence, because different coherent cells from different parts of thebeam may be mixed. The impact on vertical uniformity of an exemplarybeam is depicted in FIG. 9. Specifically, the stretched pulse 404 isplotted as intensity vs. vertical width in FIG. 9 and can be comparedwith the shape of the power amplifier output pulse 406 (unstretchedpulse) which is also plotted in FIG. 9. For the case where the beam isnear Gaussian in the horizontal axis, which is often typical when usingan excimer laser source, the impact of the pulse stretcher 22 onhorizontal intensity variations may be negligible.

As indicated above, the performance of a laser crystallization processmay be dependent on energy density uniformity. Unlike lithography whichis a multi-shot process and enjoys shot-to-shot averaging duringexposure, laser crystallization is, for the most part, a single shotprocess, and thus, averaging may be limited to intensity averagingwithin a single pulse. Some of the factors that determine energy densityuniformity are laser beam uniformity and beam spatial coherence.Typically, optics may be included in the optics module 28 (FIG. 1) tohomogenize the beam. These optics may involve the use of an array ofmicrolenses to divide the beam into beamlets. A large aperture lens maybe used to redirect the beamlets so that they precisely overlap eachother in the focal plane of the spherical lens. The integration of thesebeamlets can effectively smooth out any intensity fluctuation, yieldinga flat-top beam profile. The more beamlets the beam is divided into, thebetter the averaging may be. However, if the microlens aperture is toosmall, e.g., smaller than one coherence area of the laser beam, acoherence area may experience a repetitive pattern of microlenses, whichmay lead to undesirable results. In short, there may be a limit to theamount of homogenization achieved using an array of microlenses. Withthis in mind, averaging of the spatial coherence cells in the pulsestretcher 22 may result in a less coherent beam delivered to themicrolens array, which in turn, may minimize intensity variations due tointerference and/or permit the use of smaller aperture microlens arrays.

One feature of the pulse stretcher 22 shown in FIG. 7 that is worthy ofnote is that as the beam pointing error of the input beam (i.e., beam214B) increases, the beam divergence of the output beam (i.e., beam214C) may increase. This increase in beam divergence is oftenundesirable for laser crystallization, and accordingly, it is desirableto minimize the beam pointing error of the beam entering the pulsestretcher (i.e., beam 214B). FIG. 6 shows that an active beam steeringunit 500 can be positioned upstream of the pulse stretcher 22 tominimize beam pointing error of the beam 214B entering the pulsestretcher. This active beam steering unit may be responsive to a beampointing measurement made upstream of the pulse stretcher 22 and/or adivergence measurement made downstream of the pulse stretcher 22, forexample, a divergence measurement can be made at the SMM 26 and used tocontrol the active beam steering unit 500. Structurally, the active beamsteering unit 500 may include one or more adjustable mirrors similar tothe mirrors 240A, 240B, discussed in greater detail below and in severalapplications that have been previously incorporated by reference, toactively control beam steering in the beam delivery unit 238.Alternatively, or in addition to the active beam steering unit 500, theorientation of the beam reverser 228 can be actively adjusted to controlbeam pointing upstream of the pulse stretcher 22. Specifically, theadjustable beam reverser 228 may be responsive to a beam pointingmeasurement made upstream of the pulse stretcher 22 and/or a divergencemeasurement made downstream of the pulse stretcher 22.

FIG. 6 shows that the system 10 may comprise a beam delivery unit 24 anda stabilization metrology module (SMM 26). Functionally, these elementsmay cooperate with the laser source 20 and pulse stretcher 22 to furnisha pulsed beam at the output of the SMM 26 which meets a set of beamspecifications for the application. Indeed, the beam specifications atthe input of the optics module 28 (see FIG. 1) may depend on the designof the optics module 28 (i.e., illuminator). Specific beam parametersmay include, but are not necessarily limited to, intensity, wavelength,bandwidth, wavefront (e.g., wavefront curvature—also referred to as beamdivergence), polarization, intensity profile, beam size, beam pointing,dose stability and wavelength stability. For an optics module capable ofproducing the line beam described above for laser crystallization, e.g.,20 microns by 700 mm, it may be required to maintain pointing stabilityto within 20 μrad, wavefront curvature change to less than 10% andenergy stability to within +/−2%. Moreover, to avoid wasting shots, itmay be desirable to obtain these properties without requiring that thelaser operate continuously for a relatively long period until the laserhas “stabilized”.

The SMM 26 can be positioned upstream of an input port of the opticsmodule 28 to monitor the incoming beam and providing feedback signals toa control system to assure that the light provided to the optics module28 at the desired parameters including beam pointing, beam position,beam size, wavefront and pulse energy. For example, pulse energy, beampointing and beam position may be monitored by meteorology equipment inthe SMM 26 on a pulse to pulse basis using techniques described in U.S.patent application Ser. No. 10/425,361 (361 application) that waspreviously incorporated by reference herein. Specifically, FIG. 10B ofthe '361 application illustrates a structural arrangement for measuringpulse energy, beam pointing and beam position on a pulse to pulse basis.As detailed further below, the SMM 26 can also be configured to monitorwavefront curvature and beam size. The use of a DSP based processor,combined with high speed CMOS linear photo-diode arrays may permit rapidcalculation of beam properties at up to 8 kHz, as well as rapid feedbackto stabilize the beam properties.

The vertical and horizontal beam pointing and position errors may beevaluated at the SMM 26 for every pulse of light generated by the laser.In total there are four independent sensor measurements: verticalpointing error, horizontal pointing error, vertical position error, andhorizontal position error. In one exemplary implementation, vertical andhorizontal pointing may be measured by putting far-field images onlinear photodiode array (PDA) elements, such the S903 NMOS Linear ImageSensors offered by Hamamatsu Corporation with offices in Bridgewater,N.J. Typically, pointing errors may be defined from target locationsdefined at the exit of SMM 26. Vertical and horizontal position may bemeasured by putting reduced images of the beam near the BDU exit onlinear PDA elements. The pulse energy of the beam may be measured at theSMM 26 with a calibrated photo-cell circuit. Signals from the sensors inthe SMM 26 may be sent through electrical connectors to a StabilizationController which may form a part of the SMM 26.

Beam pointing control may be achieved by selectively adjusting theorientation of the beam reverser 228 (as discussed earlier), using anactive beam steering module 500 upstream of the pulse stretcher 22 (alsodiscussed earlier) and/or within the BDU 24. Specifically, the BDU 24may comprises two beam-pointing mirrors 240A and 240B, one or both ofwhich may be controlled to provide tip and tilt correction to vary beampointing. Beam pointing may be monitored in the SMM 26 providingfeedback control to one or both of the pointing mirrors 240A, 240B. Forexample, the error signals may be sent to the Stabilization Controllerin the SMM 26 that processes the raw sensor data and generates commandsto drive fast steering turning mirrors 40A and 40B. These two faststeering turning mirrors, each with 2 axes of control, may be placedupstream of the SMM 26, as shown. The turning mirrors can each bemounted to a fast steering motor. In particular embodiments,piezoelectric mirror drivers may be provided to permit rapid (200 Hz)beam pointing and position correction.

The motor actuates the mirror angle in two axes and thus may redirectthe path of the laser beam. Two motors with 2 axes of control can enablethe BDU stabilization controller to independently regulate the verticaland horizontal beam pointing and position errors. The control system cancorrect for the beam errors from pulse-to-pulse. Namely, the beam errorsfrom each laser pulse can be fed to a feedback control system togenerate commands for the steering motors. The electronics used to runthe feedback control system may be located in the StabilizationController. By placing the mirrors as shown in FIG. 6, the drift due tothe laser, attenuator (if provided) and other optics may be corrected.Thus, a stable beam (in position and pointing) may be projected at theentrance of the optics module 28 having, in some cases, stability towithin 10 μrad.

The pulse energy monitored at the SMM 26 may be used as a feedbacksignal and input to the laser's energy control algorithm. For a gasdischarge laser, the laser's discharge voltage may be adjusted to alterpulse energy. Since the energy control algorithm can stabilize energy atthe SMM 26 (which is at the optics module 28 input), any short term orlong term drifts in pulse energy due to optical absorption or othercauses may be compensated.

As indicated above, the SMM 26 may also measure the beam size and beamdivergence (i.e., wavefront curvature). Typically, apertures at thelaser exit can be used to fix the beam size from the laser. However,beam divergence from the laser can change due to optics heating, laserenergy, laser voltage and F2 concentration in the discharge gas whenusing a fluoride excimer laser.

As shown in FIGS. 6 and 7, beam divergence and beam size may be activelycontrolled using an adjustable beam expander 502 that can be positionedalong the BDU 24. As shown in FIG. 7, the beam expander 502 may comprisefour lenses, two horizontal lenses 504A, 504B and two vertical lenses504C, 504D. In one setup, the beam expander 502 may have a length, L, ofabout 0.30 m and be sized having a nominal input of 12 mm in thehorizontal axis and 9 mm in the vertical axis, and a nominal output of 5mm in the horizontal axis and 18 mm in the vertical axis. In anexemplary arrangement, lens 504A may be plano-convex cylindrical withf=507.0 mm, lens 504B may be plano-convex cylindrical with f=202.8 mm,lens 504C may be plano-convex cylindrical with f=202.8 mm, lens 504D maybe plano-convex cylindrical with f=405.6 mm. In an alternativearrangement, lenses 504A and 504C may be replaced by a single lens.Changes in beam divergence and beam size may be achieved by adjustingthe spacing of beam expander lenses. Specifically, the spacing betweenlens 504A and 504B may be change to alter the beam in a horizontal axisand the spacing between lens 504C and 504D may be change to alter thebeam in a vertical axis. In one embodiment, the moveable lenses can bemounted on a linear motorized drive. The expander 504 can then allowindependent control of horizontal and vertical beam wavefront.

It will be understood by those skilled in the art that the aspects ofembodiments of the present invention disclosed above are intended to bepreferred embodiments only and not to limit the disclosure of thepresent invention(s) in any way and particularly not to a specificpreferred embodiment alone. Many changes and modification can be made tothe disclosed aspects of embodiments of the disclosed invention(s) thatwill be understood and appreciated by those skilled in the art. Theappended claims are intended in scope and meaning to cover not only thedisclosed aspects of embodiments of the present invention(s) but alsosuch equivalents and other modifications and changes that would beapparent to those skilled in the art. While the particular aspects ofembodiment(s) of the Systems and Methods for Implementing an Interactionbetween a Laser Shaped as a Line Beam and a Film Deposited on aSubstrate described and illustrated in this patent application in thedetail required to satisfy 35 U.S.C. §112 is fully capable of attainingany above-described purposes for, problems to be solved by or any otherreasons for or objects of the aspects of an embodiment(s) abovedescribed, it is to be understood by those skilled in the art that it isthe presently described aspects of the described embodiment(s) of thepresent invention are merely exemplary, illustrative and representativeof the subject matter which is broadly contemplated by the presentinvention. The scope of the presently described and claimed aspects ofembodiments fully encompasses other embodiments which may now be or maybecome obvious to those skilled in the art based on the teachings of theSpecification. The scope of the present Systems and Methods forImplementing an Interaction between a Laser Shaped as a Line Beam and aFilm Deposited on a Substrate is solely and completely limited by onlythe appended claims and nothing beyond the recitations of the appendedclaims. Reference to an element in such claims in the singular is notintended to mean nor shall it mean in interpreting such claim element“one and only one” unless explicitly so stated, but rather “one ormore”. All structural and functional equivalents to any of the elementsof the above-described aspects of an embodiment(s) that are known orlater come to be known to those of ordinary skill in the art areexpressly incorporated herein by reference and are intended to beencompassed by the present claims. Any term used in the specificationand/or in the claims and expressly given a meaning in the Specificationand/or claims in the present application shall have that meaning,regardless of any dictionary or other commonly used meaning for such aterm. It is not intended or necessary for a device or method discussedin the Specification as any aspect of an embodiment to address each andevery problem sought to be solved by the aspects of embodimentsdisclosed in this application, for it to be encompassed by the presentclaims. No element, component, or method step in the present disclosureis intended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element in the appended claims is to be construed under theprovisions of 35 U.S.C. §112, sixth paragraph, unless the element isexpressly recited using the phrase “means for” or, in the case of amethod claim, the element is recited as a “step” instead of an “act”.

We claim:
 1. An apparatus for reducing beam divergence, the apparatuscomprising: a laser source configured to generate a plurality of beampulses; a pulse stretcher configured to lengthen an input beam receivedfrom the laser source by reflecting a portion of the input beam througha delay path comprising a plurality of mirrors, the pulse stretcherbeing further configured to recombine the reflected portion of the inputbeam with the non-reflected portion of the input beam; a stabilizationmetrology module configured to measure beam divergence of a beam outputfrom the pulse stretcher, wherein the stabilization metrology module isfurther configured to transmit a control signal to a beam steering unitif the measured beam divergence exceeds a predetermined threshold; andthe beam steering unit disposed before an input of the pulse stretcher,the beam steering unit comprising an adjustable mirror, wherein the beamsteering unit is configured to minimize beam pointing errors of the beamentering the pulse stretcher by adjusting the adjustable mirror inresponse to receiving a control signal from the stabilization metrologymodule.
 2. The apparatus of claim 1, wherein the laser source comprisesa two chamber excimer laser.
 3. The apparatus of claim 2, wherein thetwo chamber excimer laser has a master oscillator, power amplifier(MOPA) configuration.
 4. The apparatus of claim 3, wherein the lasersource further comprises a beam reverser.
 5. The apparatus of claim 4,wherein the beam reverser comprises a two-reflection beam reversingprism.
 6. The apparatus of claim 4, wherein the orientation of the beamreverser is actively adjustable in order to correct beam pointingerrors.
 7. The apparatus of claim 6, wherein the beam reverser isconfigured to adjust its orientation upon receiving a control signalfrom the stabilization metrology module.
 8. The apparatus of claim 6,wherein the beam reverser is configured to adjust its orientation uponreceiving a control signal from the beam steering unit.
 9. The apparatusof claim 1 further comprising a beam expander.
 10. The apparatus ofclaim 9, wherein the beam expander comprises two pairs of plano-convexcylindrical lenses.
 11. The apparatus of claim 10, wherein the spacingbetween the each pair of lenses can be adjusted to reduce beamdivergence.
 12. An apparatus for reducing beam divergence, the apparatuscomprising: a laser source configured to generate a plurality of beampulses, wherein the laser source comprises a beam reverser configured toautomatically adjust its orientation upon receiving a control signalfrom a stabilization metrology module; a pulse stretcher configured tolengthen an input beam received from the laser source by reflecting aportion of the input beam through a delay path comprising a plurality ofmirrors, the pulse stretcher being further configured to recombine thereflected portion of the input beam with the non-reflected portion ofthe input beam; and a beam steering unit disposed before an input of thepulse stretcher, the beam steering unit configured to minimize beampointing errors of the beam entering the pulse stretcher, wherein thestabilization metrology module is configured to measure beam divergenceof a beam output from the pulse stretcher, and wherein the stabilizationmetrology module is further configured to transmit a control signal tothe beam reverser if the measured beam divergence exceeds apredetermined threshold.
 13. The apparatus of claim 12 furthercomprising a beam expander.
 14. The apparatus of claim 12 wherein thebeam steering unit comprises an adjustable mirror, and wherein the beamsteering unit is configured to adjust said adjustable minor in responseto receiving a control signal from the stabilization metrology module.15. The apparatus of claim 13, wherein the beam expander comprises twopairs of plano-convex cylindrical lenses.
 16. The apparatus of claim 14,wherein the spacing between each pair of lenses can be adjusted toreduce beam divergence.
 17. An apparatus for reducing beam divergence,the apparatus comprising: a laser source configured to generate aplurality of beam pulses; a pulse stretcher configured to lengthen aninput beam received from the laser source by reflecting a portion of theinput beam through a delay path comprising a plurality of mirrors, thepulse stretcher being further configured to recombine the reflectedportion of the input beam with the non-reflected portion of the inputbeam; a stabilization metrology module configured to measure beamdivergence of a beam output from the pulse stretcher, wherein thestabilization metrology module is further configured to transmit acontrol signal to a beam expander if the measured beam divergenceexceeds a predetermined threshold; and a beam steering unit disposedbefore an input of the pulse stretcher, the beam steering unitconfigured to minimize beam pointing errors of the beam entering thepulse stretcher, wherein the beam expander comprises two pairs oflenses, and wherein the beam expander is further configured to adjustthe spacing between one or both pairs of lenses in response to a controlsignal received from the stabilization metrology module.
 18. Theapparatus of claim 17 wherein the beam steering unit comprising anadjustable mirror, wherein the beam steering unit is configured toadjust said adjustable minor in response to receiving a control signalfrom the stabilization metrology module.
 19. The apparatus of claim 18,wherein the laser source further comprises a beam reverser configured toadjust its orientation upon receiving a control signal from thestabilization metrology module.
 20. The apparatus of claim 18, whereinthe laser source further comprises a beam reverser configured to adjustits orientation upon receiving a control signal from the beam steeringunit.